docs: def export(self, nidm_version, export_dir):
          """
          Create prov entities and activities.
          """
          self.add_attributes((
              (PROV['type'], self.type),
              (NIDM_GROUP_NAME, self.group_name),
              (NIDM_NUMBER_OF_SUBJECTS, self.num_subjects),
              (PROV['label'], self.label)))